1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high powered pulse lasers, in the shock processing of solid material, and, more particularly, to a reusable gauge for measuring the strength of an energy pulse in a laser peening apparatus.
2. Description of the Related Art
Laser shock peening is a process for improving the fatigue, hardness, and corrosion resistance properties of materials by focusing radiation on preselected surface areas of a workpiece. Shock peening the workpiece can avoid gross deformation, cracking, and spallation of the workpiece, and nonplanar workpieces can be shock processed without the need of elaborate and costly shock focusing schemes.
Laser peening, or also referred to as laser shock processing, utilizes two overlays: a transparent overlay (usually water) and an opaque overlay, typically an oil based, acrylic based, or water based, black paint. During processing, a laser beam is directed to pass through the water overlay and is absorbed by the black paint, causing a rapid plasma formation and vaporization of the paint surface and the generation of a high amplitude shock wave. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece may be processed by producing a matrix of overlapping spots that cover the fatigue critical zone of the part.
It would be advantageous for maintaining control and consistency in the laser peening process to utilize a pressure gauge that would sense the pressure being applied by the laser to the workpiece. Because of the high pressure associated with the laser peening process, however, gauges currently utilized in laser peening devices are typically single use gauges, and therefore, render a relatively expensive means of determining the strength of the created shock wave.
The present invention provides a method of gauging the strength of the laser-formed shock wave with a reusable, and therefore, cost efficient device. The invention comprises replaceable components, including a pressure sensitive medium, a housing for that medium, including a screw-on ring to hold internal components in place, a cap for the pressure-sensitive medium, a back-up disk, a momentum trap, and a spring.
In the invention, a housing is removably mated with a screw-on ring, and the pressure-sensing medium is disposed between the a back-up disk and a cap. The cap includes an external surface and an internal surface, the internal surface having a convex shape for ensuring intimate contact with the back-up disk and the pressure-sensing medium.
It is an advantage of the present invention that components can easily be replaced in the gauge by opening the housing, replacing the necessary components, and assembling it back together. By this process, gauge components that wear at a faster rate than their cooperating components can be exchanged without disposing of the entire gauge.
It is a further advantage of the present invention that the gauge provides a low cost alternative to single use gauges. By providing replaceable gauge components, the invention allows a user to replace only damaged or worn out parts of the gauge, rather than the entire gauge, during the laser peening process.
The invention comprises, in one form thereof, a thick high-strength steel or ceramic back-up disk for backing up a pressure gauge medium, such as manganin. In the ceramic embodiment of the back-up disk, a circumscribing guard ring is provided for protecting the ceramic core.
A separate, thin metal disk or cap engages the back-up disk, thereby holding the pressure gauge medium in intimate contact between the back-up disk and the metal cap.
In the preferred embodiment of the invention, the pressure-gauge-engaging surface of the cap is shaped in a convex manner, so that it holds the pressure gauge medium in close registry with the back-up disk. The preferred embodiment of the invention is further defined to include alignment notches on both the cap and the housing to prevent relative rotation between the cap and the housing to prevent relative rotation between the cap and pressure gauge medium, thereby avoiding damage to the pressure medium. Adequate contact is necessary for accurately reading the strength of the shock wave. Reading of the shock wave magnitude is accomplished by the cap receiving the shock wave and conducting the shock wave to the pressure gauge medium.
The preferred embodiment of the invention further comprises a momentum trap for attenuating and trapping the shock wave, thereby preventing undue damage to the shock pressure gauge system. Without the momentum trap a shock wave could continue to reverberate within the gauge, causing fracture and yielding a shorter life span for gauge components.
Additionally, a spring is provided between the momentum trap and the housing, for holding the gauge components firmly against the back-up disk. Also, after each firing of the laser, the spring returns the components to their original position.